Feynman diagram for radiative transition

Hi,
I was just trying to draw a Feynman diagram for ψ' to [itex]χ_{c}[/itex] states.
So we have a electromagnetic transition.

I feel tempted to just have c and [itex]\bar{c}[/itex] coming in and letting a γ leave from one of the legs. But that looks like an incomplete Feynman diagram to me. Maybe I have to get two photons out?

Staff: Mentor

You need as many photons as there are photons in the final state. Feynman diagrams for electromagnetic decay of hadrons are a bit boring, as the interesting part happens in the QCD binding which you cannot include in that way.